Multi-cell batteries typically do not charge and discharge evenly because some of the battery cells are weaker than others. Unmanaged charging can result in the weaker battery cells being overcharged, and unmanaged discharging can result in the weaker battery cells being over-depleted with the stronger battery cells, and hence the battery itself, not being utilized to their full potential. For these reasons, battery management systems are typically used for implementing passive or active battery balancing during or in conjunction with charging and discharging.
Passive battery balancing entails drawing energy (i.e., “charge”) from the most charged battery cell and dissipating the drawn charge usually in the form of heat. This is undesirable during charging because the charge is drawn from the weakest battery cells, which quickly ages them. Passive battery balancing is also undesirable during discharging because charge is drawn from the strongest battery cell, which decreases discharge depth of the battery. That is, the battery lasts only as long as the weakest battery cell. Some “smart” battery management systems improve upon this by focusing on the strongest battery cell. However, this stresses the strongest battery cell and only relieves the weakest battery cell.
Active balancing entails moving charge from higher energy battery cells to lower energy battery cells. Battery cells are typically selected for balancing according to the difference between each battery cell's state of charge and the mean state of charge for all of the battery cells. However, there is a maximum amount or rate of charge that can be transferred between battery cells, which often results in the battery cells remaining unbalanced during maximum charging and maximum discharging.